Automatic light intensity control for x-ray film viewer

摘要: The intensity of light penetrating an X-ray film being viewed is automatically maintained at a preset, adjustable, eye comfort level. A photosensor detects the average level on observer side, and its output controls charging time capacitor coupled to emitter unijunction transistor. When reaches predetermined voltage it fires transistor dumps charge through pulse generator which in turn triggers gated semiconductor connected series with viewer source. period between successive firing cycles proportional Thus, if increases, as when relatively transparent negative inserted or removed, source therefore decreases restore preset penetration.